Green and blue gears have same tooth number.
OCD: straight line
AC and BD are parallel.
Triangles OAC and OBD are similar.
OC/OD = OA/OB = AC/BD = k = constant
O: immobile
Figures traced by pen E and pen C are similar. Scale factor is k.
Adjust OA and AC to get various values of k.
Bar linkage in a conventional pantograph is replaced by gear drive.
Instead of 3 gear drive a rack and two pinion drive can be used.